Ambient compensated bimetal element



March 8, 1966 D. G. WOLFE 3,238,779

AMBIENT GQMPENSATED BIMETAL ELEMENT Filed Feb. 20, 1965 FIG. 2

FIG. 4

INVENTOR DENIS G. WOLFE BY FOWLER 5 KNOBBE ATTORNEYS United States Patent 3,238,779 AMBIENT COMPENSATED BIMETAL ELEMENT Denis G. Wolfe, Youngwood, Pa., assignor to Roliertshaw Controls Company, Richmond, Va., a corporation of Delaware Filed Feb. 20, 1963, Ser. No. 260,014 8 Claims. (Cl. 73-3635) The present invention relates to an improved bimetal element and method for making same, and, more particularly, to a bimetal element which is substantially insensitive to changes of the ambient temperature.

Bimetal elements are widely used as temperature sensing means. These elements, for example, are used for making and breaking electrical switch contacts and for opening and closing fluid control valves. The electrical switch or the valve may in turn control the flow of fuel to a stove, furnace or like heat source. A particular application involves heating the bimetal element in accordance with the flow of electrical current through a given electrical circuit such as an electrical resistance heating element wound upon or proximately located with respect to the bimetal element. This application requires that the bimetal element be sensitive only to the temperature change caused by the resistance heater, or stated in another manner, the element must be insensitive to changes in ambient temperature. Such components are said to be ambient temperature compensated.

One form of ambient temperature compensated bimetallic elements known in the art is formed by butt welding reversed bimetal segments. The metal having a low coefficient of heat expansion in one segment is then abutted against the metal having a high coefficient of heat expansion in the other segment. This component is then preferentially heated by the electrical heating element wherein the heater is wound about or located proximate to only one segment of the bimetallic element and desirably has no effect upon the temperature of the other segment. The entire bimetal element is subject to the ambient temperature; however, by suitably proportioning the lengths of the reversed segments, the fiexure of one segment caused by an ambient temperature change will be compensated for by the fiexure of the adjoining reversed segment caused by the same ambient temperature change. Although this type of element provides the requisite ambient temperature compensation, it has been found difficult to isolate the control heating to only one segment since the weld readily conducts heat to the adjoining segment.

Accordingly, it is an object of the present invention to provide an ambient compensated bimetal element affording a substantial heat barrier between respective segments thereof.

Another disadvantage of the prior art ambient compensated bimetal elements is that they require fairly extensive manufacturing operations for their fabrication. For example, as noted above, one prior art element entails welding one segment to another.

It is therefore another object of the present invention to provide an improved method for making an ambient compensated bimetal element whereby it is very easily and quickly fabricated.

A further object of the invention is to provide an ambient compensated bimetal element which may be fabricated by ordinary sheet metal stamping methods.

Other and further objects, features and advantages of the invention will become apparent as the description proceeds.

Briefly, in accordance with a preferred form of the present invention, there is provided an ambient temperature compensated bimetal element comprising a unitary bimetal sheet having a longitudinally extending slit extending from one end there-of to within a short distance from the other end. A pair of legs are thus formed which are joined by a relatively narrow web. These legs are then bent in opposite directions perpendicular to the web thereby forming respective reversed bimetal segments. These segments are joined by only the thin web which provides a poor heat conduction path between the mutually reversed segments.

Bimetal elements constructed in the manner described above and in more detail below offer a very simple but effective solution for the two problems imposed by the prior art devices. Thus, their construction inherently affords a substantial heat barrier between these segments and furthermore, the elements may be constructed by utilizing ordinary sheet metal cutting and stamping techniques. No Welding or other joining of segments are required, the entire element being formed by cutting and folding a unitary sheet of bimetal material.

A more thorough understanding of the invention may be obtained by a study of the following detailed description taken in connection with the accompanying drawing in which:

FIG. 1 is a perspective view of an ambient temperature compensated bimetal element constructed in accordance with the present invention;

FIG. 2 illustrates the bimetal sheet after being slit but before bending thereof;

FIG. 3 is a side elevation of the invention prior to a change in temperature;

FIG. 4 is a plan view of the invention;

FIG. 5 is a side elevation view of the invention following a change in ambient temperature; and

FIG. 6 illustrates the bimetal element of the invention employed in an electrical switch. 1

Referring now to FIGS. 1 through 4, the invention comprises a flat sheet of bimetal material 10 which normally includes two continuous sheets of metal 11, 12 having widely different coefficients of heat expansion which are joined together face-to-face. One of these metals is ordinarily Invar steel and the other an alloy including for example nickel or manganese. This sheet is cut as shown in FIG. 2 to form a longitudinally extending slit 13. Respective legs 14, 15 joined by a relatively narrow web 16 are thereby formed as shown in FIG. 2. Each of the legs is bent in opposite directions through an angle of so as to lie in a common plane perpendicular" to the web 16 (FIG. 3). There is thus formed a bimetal element composed of mutually reversed segments, that is,

metal 11 is on the upper surface of segment 14 whereas it is on the lower surface of se ment 15. The web 16 may be given a longitudinal bend 17 substantially perpendicular to the longitudinal axis of segments 14, 15 to impart additional structural rigidity to the member. This bend thus resists twisting of the component about its longitudinal axis.

There is thus formed by the simple expedient of cutting and folding a single bimetal sheet, a bimetal element having reversed segments. It will be evident from the figures and particularly FIG. 4 that each of the segments 14, 15 are joined by only a narrow web 16. This construction affords a substantial heat barrier between the segments since all of the heat must flow through this narrow strip in order to pass from one segment to the other. This component provides the desired ambient compensation as shown in FIG. 5 wherein a change in ambient temperature causes a flexure of segment 14 opposite that of the flexure of segment 15. The proportionate lentghs of the legs will determine the desired movement ratio FIG. 6 illustrates the use of the ambient compensated bimetal element of the present invention in a typical application wherein electrical contacts 20, 21 are desired to be closed in response to heating of resistance heater coil 22 wound upon segment 14 of the bimetal element. Seg' ment 15 is anchored to support 23 to suspend the bimetal element in cantilever fashion.

In operation of the switch FIG. 6 is as follows: an electrical current passing through heater coil 22 causes preferential heating of segment 14 with respect to segment 15 and closure of the contacts 20, 21. The poor heat conductivity path provides an effective heat barrier for the heat generated by coil 22 so the bimetal segment 15 is affected very little thereby. If there is a change in ambient temperature affecting both of the segments, the segments flex as shown in FIG. without changing the relative position of electrical contact 20 with respect to fixed electrical contact 21.

Although exemplary embodiments have been disclosed and discussed, it will be understood that other applications of the invention are possible and that the embodiments disclosed may be subject to various changes, modifications and substitutions without necessarily departing from the spirit of the invention.

I claim:

1. An ambient compensated bimetal element comprising:

a unitary bimetal sheet having a longitudinal slit extending from one end thereof to near the opposite end thereof, said slit forming a pair of legs joined by a relatively narrow web,

said legs extending in substantially opposite directions from said web to form reversed bimetal segments.

2. An ambient compensated bimetal element comprisa unitary bimetal sheet having a longitudinally extending slit therein forming a first leg and a second leg joined by a relatively narrow web,

said legs being bent relative one another to form mutually reversed bimetal segments which are substantially coplanar,

said web being sufficiently narrow to provide a low heat conductivity path between said segments.

3. The ambient compensated bimetal element defined in claim 2 wherein:

said web lies in a plane substantially perpendicular to the plane defined by said legs and further comprising means for stiffening said element against twisting about its longitudinal axis.

4. A bimetal element responsive to a preferential heating of one segment thereof but substantially insensitive to heating of both a first and second segment thereof which comprises:

a bimetal sheet including a continuous sheet of metal having a first coefficient of heat expansion and a second sheet of metal having a higher coefficient of heat expansion joined together face-to-face and having formed therein a longitudinal slit extending from one end of said bimetal sheet to near the opposite end thereof,

said slit thereby forming a pair of legs joined by a relatively narrow web,

said legs being bent relative one another to form mutually reversed bimetal segments wherein the sheet having a low coefficient of heat expansion in one of said segments occupies the position corresponding to the sheet having a high coefficient of heat expansion in the other of said segments,

said web being sufficiently narrow to provide a low heat conduction path between said segments;

means for anchoring one of said segments to thereby support said member in cantilever fashion; and

means actuated by said bimetal element coupled to said other segment.

5. An ambient compensated bimetal element compris- (a) a unitary bimetal sheet having a longitudinally extending slit therein forming a first leg and a second leg joined by a relatively narrow web;

(b) said legs being bent relative to one another to form mutually reversed bimetal segments;

(c) said Web including an additional longitudinal bend along a line substantially perpendicular to the longitudinal axis of said first and second segments for stiffening said element against twisting about its longitudinal axis; and,

(d) said web being sufficiently narrow to provide a low heat conductivity path between said segments.

6. A bimetal element responsive to a preferential heating of one segment thereof but substantially insensitive to heating of both a first and second segment thereof which comprises:

(a) a bimetal sheet including a continuous sheet of metal having a first coefficient of heat expansion and a second sheet of metal having a higher coefficient of heat expansion joined together face-to-face and having formed therein a longitudinal slit extending from one end of said bimetal sheet to near the opposite end thereof;

(b) said slit thereby forming a pair of legs joined by a relatively narrow web;

(0) said legs being bent relative one another to form mutually reversed bimetal segments wherein the sheet having a low coefficient of heat expansion in one of said segments occupies the position corresponding to the sheet having a high coefficient of heat expansion in the other of said segments;

(d) said web being sufficiently narrow to provide a low heat conduction path between said segments.

7. The method of making an ambient compensated bimetal element which comprises the steps of:

cutting a longitudinally extending slit in a bimetal sheet to form first and second legs joined by a web and bending each leg in an opposite direction through an angle of approximately so as to form mutually reversed bimetal segments.

8. The method of forming an ambient compensated bimetal element comprising:

(a) slitting a bimetal sheet to form first and second legs joined by a relatively narrow web;

(b) bending said legs to form mutually reversed bimetal segments; and,

(c) bending said web along its longitudinal axis for stiffening said element against twisting about its longitudinal axis.

References Cited by the Examiner UNITED STATES PATENTS 1,710,782 4/1929 Mottlau 73-363.5 1,938,929 12/1933 Peterson 73-363.5 X 2,144,589 1/1939 Rich 73-3631 X 2,284,383 5/1942 Elmer 73-363.5 X 2,572,059 10/1951 Schlaich 73363.1 X

DAVID SCHONBERG, Acting Primary Examiner.

ISAAC LISANN, LOUIS R. PRINCE, Examiners.

D. M YASICI-l, Assistant Examiner, 

1. AN AMBIENT COMPENSATED BIMETAL ELEMENT COMPRISING A UNITARY BIMETAL SHEET HAVING ALONGITUDINAL SLIT EXTENDING FROM ONE END THEREOF TO NEAR THE OPPOSITE END THEREOF, SAID SLIT FORMING A PAIR OF LEGS JOINED BY A RELATIVELY NARROW WEB, SAID LEGS EXTENDING IN SUBSTANTIALLY OPPOSITE DIRECTIONS FROM SAID WEB TO FORM REVERSED BIMETAL SEGMENTS. 